The telescope is situated atop the inner of two concentric towers
attached to the Martin A. Pomerantz Observatory (MAPO) at the South
Pole station (see Figure 1); the inner tower is mechanically isolated
from the outer tower, so that no vibration can be transmitted from the
building to the telescope. A room beneath the telescope,
vibrationally isolated from the telescope, houses helium compressors,
drive amplifiers, and an air handling unit for managing waste heat
from the telescope and helium compressors. The interior of the
telescope opens directly onto the compressor room, providing access to
drive systems, receivers and electronics even in mid-winter, when the
darkness and extreme cold (as low as C ambient) severely
restrict outside activity. An insulated fabric bellows allows motion
in the elevation axis while keeping the interior of the telescope and
drive assemblies at room temperature.

Figure:The DASI telescope (left) being lifted to the 35-ft tower attached to the
MAPO building at the Amundsen-Scott South Pole Station 3 December 1999
and (right) operating at sunset March 2000.

The telescope consists of an altitude-azimuth mount, employing a
counterbalanced gear and pinion elevation drive, yielding excellent
tracking and pointing stability. Heavy box steel-plate construction
lends the mount extreme rigidity and immunity to flexure. The
interferometer has 13 primary antenna elements, arranged in a
three-fold symmetric pattern on a rigid faceplate, which attaches to
the elevation cradle.

The faceplate can also be rotated about its axis; in combination with
the three-fold symmetry, this feature provides important diagnostic
capabilities, for instance permitting discrimination of spurious
signals due to cross-talk between the antenna elements, a concern with
any compact array in which the elements are nearly touching. Since
the antenna pattern repeats with every
of rotation, any
signal in the far field will be unchanged by the rotation, while any
signal due to cross-talk will rotate with the faceplate. For purposes
of imaging, the rotation also allows dense sampling of the u-vplane.

In addition, the rigid faceplate greatly simplifies the design of an
interferometer. Unlike conventional tracking arrays, projected
baseline lengths for a co-planar array are independent of the pointing
center, and IF tracking delays are not required.